This application claims the benefit of European Application No. 00400816.5, filed Mar. 23, 2000.
This invention relates generally to planar optical devices. In particular, the invention relates to optical devices such as an optical switching array that uses a variable filter element, or a mirror, immersed within an optical planar device.
Planar optical devices are known where at least an integrated optical waveguide is fabricated as one or more layers of waveguide material (such as silica, a dielectric, a thin film, a polymer, a sol-gel) deposited on an insulator or other substrate using micro-electronic techniques such as vapor deposition, sputtering, or epitaxial growth, or a micro-replication technique such as embossing or printing, and patterned using techniques such as photolithography, e-beam lithography, or micro-molding. Such a waveguide conventionally having a core embedded between a solid underclad layer and a solid overclad layer is often used as a first substrate for interfacing with another substrate or hybridized with another electrical component.
Such planar fabrication techniques have evolved to form very special types of optical devices, such as micro-electro-mechanical system (MEMS) optical devices having specialized optical features or configured for various optical functions.
As one example, switch arrays with movable micro-electro-mechanical system (MEMS) mirrors or other types of variable filters used to direct the propagation of light at the cross-points of waveguides are one type of such known MEMS substrates.
One fabrication approach uses an index-matching fluid as the switching element for a planar MEMS switch. A planar waveguide array is formed on a first planar substrate. Trenches are formed at the cross-points and are filled with a fluid that matches the refractive index of the waveguide core. In order to actuate the switch, the fluid is either physically moved in and out of the cross-point using an actuator, or the fluid is thermally or electrolytically converted into a gas to create a bubble. For this approach to work, the facets cut at the end of the waveguide at the cross-points must be of mirror quality, since they are used to reflect the light into the desired waveguide. Finally, the fluid must be withdrawn cleanly to preserve the desired facet geometry and to prevent scattering or beam spreading losses due to any remaining droplets.
In another approach, a beam from a second substrate, referred to as the MEMS substrate, is disposed diagonally over a gap in a waveguide of the first substrate. A mirror from the MEMS substrate is suspended from the beam into the gap of the first substrate. An electrode is disposed adjacent to the gap and underneath the beam. When the electrode is addressed, the beam and mirror move into the gap to reflect light propagating in the waveguide. This method is also subject to beam-spreading problems such that the typical losses from such a switch would be high.
Often, the MEMS substrate and the waveguide planar substrate which supports the MEMS substrate will be fabricated separately and then assembled together. The assembly technique is thus constrained by the height of the mirror and the depth of the trench in the waveguide planar substrate. Conventionally, the maximum depth of the trench is set by the etching process limitation and the minimum by the solid overclad thickness required for a low-loss optical propagation in the waveguide. The height of the MEMS mirror is also determined by the fabrication process. Hence, in actual implementation, the maximum gap between these two substrate is 5 xcexcm. The assembly technique for interfacing these two substrate must fit in this gap. However, standard flip-chip solder bump technology requires a height of 10 xcexcm and thus cannot be used with a conventional waveguide structure having an overclad as the first substrate. Because the assembly must also provide an electric connection between corresponding pads of the two substrates, standard glue cannot be used. Conductive glues are usually thick, above 20 xcexcm. Generalizing this assembly constraint to other optical devices, the problem to be solved requires a reliable assembly technique at the interface of any two substrates which have to be electrically connected without suffering an optical loss.
A need therefore exists for better planar fabrication or assembly techniques to optimize functions of optical devices such that, for example, a minimal loss optical switch having splitting or variable switching features can be easy and reliable to assemble at the interface of the two substrates.
The present invention addresses the needs discussed above. A planar optical device and its method of fabrication includes a planar substrate on top of which is deposited a layer of waveguide material. A waveguide circuit is etched into the waveguide material. A cover selectively encapsulates the substrate around the waveguide circuit while leaving space to provide a gap above the waveguide circuit. A liquid material having a lower index of refraction than the waveguide material is filled into the gap above the waveguide circuit such that the liquid acts as an overclad for the waveguide circuit.
One aspect of the present invention is that an optical device includes an optical path characterized by a refractive index, for propagating a light signal. An optical modifying element is disposed in the optical path for modifying the light signal. An overclad liquid having a refractive index less than the refractive index of the optical path is then disposed between the optical path and the optical modifying element for reducing optical loss of the light signal.
Another aspect of the present invention is that the optical modifying element can be another substrate to be interfaced with a first substrate.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.